1. Field of the Invention
The present invention relates to systems for attitude control of Earth-orbiting satellites and, more particularly, to yaw steering control of a satellite that maintains programmed sun pointing for solar power generation.
2. Prior Art
Systems for satellite motion control, and in particular for dual-orientation orbital attitude control, are known, such as disclosed in U.S. Pat. No. 5,791,598. These systems may be used for controlling the solar fixation of a nadir-pointing satellite that is employed in communications systems, e.g., the GLOBALSTAR.TM. communications satellite system. In general these systems employ a bias momentum scheme which provides gyroscopic stability. As noted in the patent, attitude control of satellites is straight forward when three-axis sensor information is available, such as 1) an earth horizon sensor providing pitch and roll information, and 2) a sun sensor measuring yaw. However, during the solar-eclipse that occurs each time a satellite orbits the earth, such sun-sensor data is not available. This results in a gap in the yaw sensing information during each orbiting period.
Supplemental sensors can be added to the satellite, but this still does not assure continuous yaw information. For example, a magnetometer can be used to supply supplemental yaw data. But, as the satellite orbits through the earth's higher latitudes, the earth's magnetic pole tilts toward the satellite and the satellite's magnetometer data becomes too inaccurate to provide adequate attitude control during magnetic eclipses that occur at such latitudes. In addition, for high altitudes, including the geo-synchronous altitude, magnetic attitude references are unusable.
Major challenges presented in designing communications satellites, such as the GLOBALSTAR.TM. communications satellite system, is 1) the need to operate without continuous yaw information while providing continuous controlled-yaw motion, and 2) the need for particularly critical and complex attitude control. The communications antennas of such satellites are conventionally mounted on the portion of the satellite that is always nearest to the earth, which is referred to as a "nadir-pointing" attitude, and this attitude must be maintained at all times. Attitude control is particularly critical for communications relay satellites, such as those used in the GLOBALSTAR.TM. satellite cellular phone system, because they also have very high power requirements. Efficient operation of the solar panels on these satellites requires that the sun's rays be normal to the plane surface of the solar panels at all times. Therefore, reliable, precise satellite attitude control is essential to the GLOBALSTAR.TM. satellite system's mission and certain systems that have been proposed and implemented may be used to this end. Exemplary systems for this purpose include the following.
a. Whecon Stabilization
Momentum-bias attitude stabilization systems have successfully been used to provide precise attitude stabilization for fixed-orbit nadir-pointing satellites, without direct yaw sensing. The Wheel-Control or "Whecon" system, described in the article "Analysis and Design of Whecon--an Attitude Control Concept", by H. J. Dougherty, E. D. Scott, and J. J. Rodden, AIAA paper no. 68-461, AIAA 2nd Communications Satellite Systems Conference, San Francisco, Apr. 8-10, 1968, is an example of such a system. The Whecon bias-momentum system provides three-axis satellite stabilization in response to pitch and roll signals from an earth-horizon sensor. Whecon then controls the residual yaw errors through a dynamic coupling of yaw with the orbital pitch rate.
The Whecon system uses: 1) a momentum wheel having a fixed alignment with the spacecraft pitch axis; 2) horizon sensors that detect pitch and roll attitude errors; and 3) mass expulsion devices for responding to those errors, all without direct yaw sensing. However, the momentum wheel's inertia restrains vehicle yaw rotations to small perturbations about zero. This attitudinal rigidity makes the Whecon system inapplicable to solar-powered communications satellite systems that have orbits that precess, such as the system. Also, mass expulsion systems use non-renewable energy sources that limit the working life of the satellite.
The use of momentum for attitude control, instead of mass expulsion engines, is a concept that is attractive for its simplicity. However, this type system, which was used on a number of U.S. and international synchronous satellites, including military satellites, Intelsat V, and the Canadian Communication Technology Satellite, also uses mass expulsion to provide control torque.
b. Seasat Nadir-pointing Momentum Bias
This all-momentum-wheel version of the Whecon attitude stabilization system was developed for the NASA-JPL Seasat satellite, which flew in 1978. This system is described in the article "Seasat A Attitude Control System", by R. Weiss, J. J. Rodden, R. W Hendricks and S. W. Beach, pp 6-13, Journal of Guidance and Control, vol. 1, no. 1 (1978). The Seasat platform uses momentum wheels for the orbital momentum bias necessary to maintain a nadir-pointing attitude, as well as for vehicle stabilization, instead of using mass expulsion to produce the bias momentum.
Attitude sensing is performed by a pair of "scan" wheels on the satellite. Seasat provides magnetic compensation for the momentum wheels, referred to as "desaturation", to counter the momentum produced by sources of torque that are external to the vehicle, including the earth's gravitational and magnetic fields.
Seasat solar orientation is monitored by a pair of sun-aspect sensors. However, these sun-sensors are not used for orbital attitude control in the Seasat, since Seasat was designed as a sun-synchronous earth satellite. Sun-synchronous earth satellites have a fixed relation to the sun so that such vehicles do not need yaw-steering.
c. GLOBALSTAR's Asynchronous Precession
Solar-powered satellites in orbits that precess, but are not sun-synchronous, can use either a combination of yaw-steering and rotary solar panel motion, or compound motion of the solar panels to track the sun. A nadir-pointing satellite could, alternatively be articulated between transmitter end and solar panels mounted at its opposite end to permit the two ends of the satellite to twist relative to each other, as was done by the Seasat platform. However, this requires the power from the panels to be supplied to the transmitter through an additional inefficient, failure-prone connector, such as a wiper assembly, or the like. This is unacceptable for high-powered, high-reliability relay satellites. FIG. 3 shows the yaw-steering motion needed to keep the solar panels of a sun-tracking solar-powered satellite continuously facing the sun at an optimal angle as the satellite orbits the earth. The solar array is rotated about a y-axis of the vehicle through the angle "SADA", and the vehicle is yawed through the angle ".psi.". The magnitude of the yaw excursion variable .psi. depends on the angle between the sun and the orbital plane, the angle .beta.. In the limiting case that occurs when the sun is in the plane of the satellite's orbit, .beta.=0 and only the SADA angle is variable. No yaw motion is needed. At higher .beta. values both SADA and yaw must vary.
For the satellites of the GLOBALSTAR.TM. system, the precession of their orbits produces a beta angle ".beta." between the satellites'orbital planes and a line from the earth to the sun that varies from zero to about 75 degrees. At .beta.=75.degree., a satellite must provide very large rotations of the solar array about the nadir-pointing z-axis and perpendicular to the pitch axis, and a very large SADA angle about the y-axis, to maintain solar-power efficiency.
If yaw-steering is not used, the secondary deflection of the solar array about that z-axis perpendicular to the pitch axis must be reduced, to prevent one of the solar panels from crossing into areas where they reduce the operating efficiency and coverage pattern of the satellite's relay operation each time they reach maximum yaw deflection, by interfering with the satellite's antennas.
This limit on the compound motion of the solar panels causes less-than-optimal sun tracking. Less-than-optimal sun tracking by the panels requires a substantial increase in their size, to offset the resulting decrease in efficiency. Very large arrays, however, are subject to gravity gradient perturbation effects, as well as being more expensive and massive. Thus compound motion of the solar panels is not a satisfactory solution of the problem.
If the GPS satellites' attitude control system could be adapted to provide the precision yaw-steering necessary for the system, both the satellite's structural integrity and the effectiveness of its antenna system would be preserved. However, this kind of precise attitude control requires a second, very accurate, continuously-controlled angular rate offset, so that the satellite's attitude points the solar panel normal to the sun instead of moving the solar panels themselves or twisting the satellite amidships, i.e., between the antenna and the solar panels.
While the use of momentum devices for both attitude sensing and attitude correction is a concept that is attractive for its simplicity, reliance upon momentum sensing rate-gyro instruments for attitude sensing is impractical because they are imprecise, as well as expensive. Also, rate gyro sensors are too susceptible to drift, which is especially problematic when a satellite is subject to a significant variation in the torques applied to it, as are satellites that provide yaw-steering. United States Patent, "Dynamic Bias For Orbital yaw Steering", U.S. Pat. No. 5,791,598, dated Aug. 11, 1998 enables similar yaw steering capability but its performance is limited to satellites with relatively small inertia properties and is applicable only to nearly circular orbits. The current submission applies to satellites with large inertia and differences in principal inertia mass distributions. The submission also can operate satisfactorily in highly elliptic orbits.